1. Field of the Invention
The present invention relates to an image capturing control technique.
2. Description of the Related Art
In a digital camera including an image sensor, image sensor sensitivity (hereinafter also called “imaging sensitivity”) can be generally changed. For example, when assuming that an inherent sensitivity of the image sensor is a minimum sensitivity, a sensitivity equal to or higher than the minimum sensitivity can be realized by amplifying the output of the image sensor. More specifically, when the inherent sensitivity of the sensor is equivalent to an ISO sensitivity of 100, an ISO sensitivity of 200 can be realized by amplifying the output of the image sensor twofold, and an ISO sensitivity of 400 can be realized by amplifying the output of the image sensor fourfold.
Since the amplification of output by such a sensitivity change results in amplification of not only an image signal but also noise included in the image signal, when the imaging sensitivity is set to an ISO sensitivity of 200, the amount of noise doubles compared with a case in which the imaging sensitivity is set to an ISO sensitivity of 100.
When the imaging sensitivity is high, the output of the image sensor is amplified; therefore, the exposure time is short. On the other hand, when the imaging sensitivity is low, the exposure time is long. Therefore, when performing exposure for a relatively long time, such as when capturing an image of a night view or a celestial body, the exposure time largely varies depending on the set ISO sensitivity. For example, a subject which typically has to be exposed for one minute at an ISO sensitivity of 100 can be photographed with an exposure time of 7.5 seconds at an ISO sensitivity of 800.
As described above, since the imaging sensitivity affects exposure time and noise included in a captured image, it is preferred that, when capturing an image, the imaging sensitivity is appropriately set to shorten exposure time, while suppressing the amount of noise to an acceptable level. In other words, when capturing an image, the imaging sensitivity is preferred to be set considering the balance between the exposure time and the amount of noise.
The imaging sensitivity is set automatically in accordance with the use of ambient light or flash light, or set manually by a photographer on an experimental basis.
In a normal photographing operation, the photographing operation is completed instantly. Therefore, it is relatively easy to perform a photographing operation again if the imaging sensitivity is determined to be inappropriate after checking the captured image by using a display section such as an LCD after capturing the image. However, in a case of performing a photographing operation with a long exposure time (also called “long time exposure image capturing”), the time necessary for the photographing operation is long, so that a wait time until the captured image is checked using the display section is long. When performing a photographing operation again due to an inappropriate imaging sensitivity, more time is consumed. Therefore, it is very troublesome for a photographer who is not sure about the set imaging sensitivity to perform long time exposure image capturing.
As a technique which deals with the problems associated with long time exposure image capturing, there is a technique in which an image being photographed is displayed by a display section at any time during the exposure time, so that the image can be checked in long time exposure image capturing (for example, Japanese Unexamined Patent Application Publication No. 2003-69897). By using this technique, changes in an image being photographed which occurs as the exposure time passes can be checked. Therefore, it is possible to determine whether the exposure amount is sufficient or not during exposure.